A speed trap detection and warning system for a vehicle (9) comprises a GPS or mobile telephone signal receiver and processor which calculates the location of the vehicle (9), the speed thereof and the direction of travel thereof, a memory storing data defining the location of a number of speed traps (5, 6) and alerts the driver if the vehicle is within a predetermined distance of a speed trap and the vehicle's speed is greater than the road speed limit. Also disclosed is a system for calculating the location and speed of a vehicle (9), comparing the calculated data with stored data defining the locations of roads and the speed limits therefor, and alerting the driver if the vehicle's speed exceeds the speed limit for the road on which it is travelling. A further system is disclosed for displaying to the driver the speed limit for the road on which the vehicle is travelling.